My overweight roomates are a negative influence?

Well, you stated it many times it is not their fault. But their way of life is rubbing off onto you, it is not wrong in anyway, but currently it is not making you comfortable, otherwise you wouldnt have complained about it. So you are yourself going to commit to your own life style. When they say break-time, tell them you are already having coffee for the day, enjoy the break but stay away from the cookie/sugary coffee. When they cook and hand it out to you on a big plate, instead, next time, pick a smaller plate for yourself and adjust your portion. Nobody makes you eat excessively , the reason why you are eating a lot because you let yourself to be so. Now of course living with so many people where your habit may come at cross purpose with the rest of them can be a bit hard, but hey you just need to stand your ground, stay healthy is a wise decision. You still have an option to exercise which benefits your health and you may even get them to join in, group exercise is fun and productive. If they dont want to lose weight, well then they make their choices, its time to make yours.